The present invention relates to a lubricant containing a perfluoropolyether compound, and a magnetic recording medium using the same.
To meet the increasing demand for small size, large capacity hard disk drives, researches and developments for high density recording are under way. High density recording with a head and disk system is achievable if the distance between a head and a disk is reduced and if the spinning speed of the disk is increased. This, however, increases the probability of high-speed contact of the disk with a slider supporting the head and thereby deteriorates a lubricant, resulting in various kinds of troubles. I found, based on a series of studies and experiments, that a lubricant deteriorates generally in two different modes, one ascribable to the decomposition of perfluoropolyether contained in the lubricant and the other ascribable to the generation of a highly viscous substance.
The decomposition of perfluoropolyether is generally classified into thermal decomposition and chemical decomposition. Thermal decomposition is caused by frictional heat ascribable to the high-speed contact of the slider and disk. Chemical decomposition is ascribable to chemical reaction between perfluoropolyether and Al.sub.2 O.sub.3 --TiC constituting the slider or acids derived from perfluoropolyether. A lubricant capable of reducing the thermal decomposition of perfluoropolyether and the chemical decomposition ascribable to the reaction of perfluoropolyether with Al.sub.2 O.sub.3 --TiC has not been reported in the past.
A lubricant capable of reducing the decomposition of perfluoropolyether ascribable to its reaction with acids has been proposed in various forms. For example, perfluoropolyether with tri-alkylamine added thereto is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 5-20675, 6-145687, 7-62738, 7-93744, and 7-93745. Among them, Laid-Open Publication No. 5-20675 relates to a lubricant with tri-alkylamine added to perfluoropolyether having a hydroxyl group at its end. This document teaches that tri-alkylamine may be either one of straight chain type and branched type, but should preferably have six or more carbons, and that such a lubricant implements desirable lubrication, e.g., smooth running, wear resistance and durability over a long period of time. I actually confirmed that so long as this kind of lubricant is applied to audio tapes, video tapes and other magnetic tapes, the decomposition of perfluoropolyether is slow enough to enhance the performance including smooth running.
However, when it comes to advanced hard disk drives causing disks to spin at a speed of 7,200 rpm (revolutions per minute) or above, none of the conventional lubricants including one taught in Laid-Open Publication No. 5-20675 can reduce the decomposition of perfluoropolyether or prevent the lubricating ability from being lowered.
Why the lubricant containing perfluoropolyether having a hydroxyl group at its end and tri-alkylamine added thereto preserves desirable lubrication over a long period time is presumably as follows. The lubricant neutralizes, with tri-alkylamine, acids (mainly carboxylic acid) generated therein by thermal decomposition ascribable to sliding and playing the role of a catalyst for the decomposition of perfluoropolyether, thereby cancelling the catalytic action. However, in the case of a hard disk, acids (mainly hydrofluoric acid) to be generated by sliding are different from acids derived from a magnetic tape and cannot be fully neutralized by alkylamine. Moreover, the acids particular to a hard disk are several hundred to several ten thousand times as high in density as the acids particular to a magnetic tape.
Truly effective means for reducing the decomposition of perfluoropolyether under the above severe conditions has not been reported yet. Further, an implementation for reducing the decomposition of perfluoropolyether ascribable to its reaction with Al.sub.2 O.sub.3 --TiC is now known at all.
The highly viscous substance derived from the deterioration of the lubricant is a more critical cause of troubles between the head and the disk. This kind of substance is a denaturated substance mainly constituted by a complex compound of a decomposed substance derived from perfluoropolyether and metal ions. Cobalt which is the major component of a magnetic recording layer is present, although in a small amount, on the protection layer of a disk, as well known in the art. Such cobalt is ionized by the hydrofluoric acid derived from perfluoropolyether, causing the complex compound formation to proceed. Should the viscous substance deposit on a slider, it would cause the aerodynamic characteristic and therefore the float, pitch angle and roll angle of the slider to change. This aggravates the error rate at the time of recording or reproduction and the probability of high-speed contact of the slider and disk, and causes the slider edge and disk to slide on each other at a high speed, bringing about head crashes in a short period of time.
While the above problems may be effectively solved by a lubricant of the kind generating no highly viscous substances despite the high speed contact of the slider and disk, such a lubricant is not known in the art.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 61-113130, 5-217152, 7-182652, and 8-83423.